1. Field of the Invention
The present invention relates to generating a linearly modulated signal using phase shift keying, which can tolerate distortion in transmission, and achieving optimal or near optimal demodulation performance in the presence of distortion. The distortion can be caused by RF circuits including high power amplifiers and up-converters and mixers at radio frequency or by the channel. The invention may have applications in communications systems using phase shift keying, wireless communications, satellite communications, radar, and in particular in a communication terminal or radar which needs enough transmission power, or high DC-to-AC power conversion efficiency, or simple design for transmitter or receiver, or simple radio frequency circuit design.
2. Background and Description of Related Art
In recent years the data rate demanded for wireless communications has increased dramatically. Commercial products supporting 1 Gbps are already in service using millimeter wave and BPSK in ultra-broadband wireless communications in the last-mile. The data rate in wireless LAN is also increased significantly. The ultra-wideband (UWB) technology is under intense study for commercial applications or radar systems. It is desirable to create low complexity transceivers to support very high data rates or chip rates. The transmitter must have enough transmission power. For mobile terminals and transceivers operated on battery such as in wireless sensor networks, it is preferred to have high DC-to-AC power conversion efficiency. The high DC-to-AC power conversion efficiency can be achieved using the class-C, class-D, class-E or class-F power amplifiers. These power amplifiers cause non-negligible distortion to linear phase modulations and are traditionally not used in systems which employ non-constant envelope phase modulations such as phase shift keying. The transmitter or the receiver needs simple implementation including simple RF circuit to minimize the cost. The system needs high bandwidth efficiency and optimal or near optimal demodulation performance.
Constant envelope modulation methods have been widely used to tolerate distortion. This was achieved by keeping the envelope of the modulated signal as constant and embedding the information in the phase or the frequency of the modulated signal. A popular subset of constant envelope modulations is known as continuous phase modulation. The complexity in either the transmitter or the receiver is usually not low for constant envelope modulations. An equalizer is needed in the receiver to achieve optimal or near optimal demodulation performance. Because of the complexity, constant envelope modulations are not preferred when data rate or chip rate is high.
Herbst et. al. (as represented by U.S. Pat. No. 5,812,604) have invented a method to form constant envelope OQPSK signal using hard-limiting. The constant envelope OQPSK signal is easier to be up-converted to radio frequency.
Inspired by the results in the constant envelope OQPSK, which was independently obtained in 1996 by the inventor of this application, Jackson and Roos invented the implementation of the constant envelope OQPSK using MSK modulator. The implementation is represented by U.S. Pat. No. 6,301,310, which cannot give good power spectral density and causes large degradation in signal-to-noise power ratio for bit error performance.
In summary, previous methods have practiced generating constant envelope. They followed the traditional theory and practice that constant envelope signals can tolerate radio distortion.